When building a cellular system the first thing to focus on is to have sufficient number of base stations to create the wanted coverage area. When the intended area is covered and the traffic starts to grow the system has to be expanded in capacity. The first step in doing this is to add more transceiver units (TRUs) to the existing base stations and to re-use the frequencies in the system as tight as possible. Due to the fact that the frequency spectrum allowed to be used by each operator is limited, this adding of TRUs can only meet the increased capacity demand to a certain extent. In the longer run congestion in the system will occur and the perceived quality in the system will be too low. Then new smaller sites have to be built to take care of the increasing capacity; i.e. a so-called cell split is needed.
Now the question arises where to build the new site. Of course it shall be built as close as possible to where the traffic is generated in order to efficiently “off-load” the congested cells. Building the cell there will also give a system with higher quality since the calls will experience higher possible signal strength enabling enough margins to possible interfering calls. Collecting the position from different mobiles using position methods available in a mobile system e.g. cell ID/TA and E-OTD based on GPS and triangulation methods give the operator a hint of where the calls are made in the systems. Using this information the operator will have a decent view of where to build the new site. A method of determining traffic density within cells for a similar purpose based on power level and geographic distribution of the traffic is described in U.S. Pat. No. 5,828,962 assigned to France Telecom.
In modern 2,5G & 3G systems the users share resources from a common resource pool. In addition different user-services consume more or less of these common resources. For example in a WCDMA system the total power that can be transmitted in a cell is limited and a subscriber using the 384 kbps packet data service will consume much more power than a subscriber making a voice call, typically around 15 kbps or even less, from the same position.
If the 384 kbps user is close to the cell border almost all available power in the cell is needed to support that connection. But if the 384 kbps user is close to the site, thereby experiencing high signal strength, the power control algorithm will reduce the transmitted output power for this connection, which will free resources from the resource pool and make room for more users in the cell. This shows the necessity of locating the new site within an area, where many users of high bit-rate services, can be served as efficient as possible.